Process for drying difluoromethane

ABSTRACT

Process for drying wet F32, which comprises placing a stream of the said F32 in continuous contact with a feed stock of a composition comprising a molecular sieve chosen from a 3A type sieve, at a temperature of between 5 and 78° C. and at a pressure of between 0.5 and 25 atm.

[0001] The present invention relates to the field of fluorohydrocarbonsand its subject is, more particularly, a process for the continuousdrying of wet difluoromethane (CH₂F₂), using a molecular sieve, of Atype, and which can be used in an industrial production plant.

[0002] Difluoromethane (known in the art by the abbreviation F32 orHFC-32) is one of the possible replacements for chlorofluorocarbons(CFC) with which the Montreal Protocol is concerned. It is moreparticularly intended to replace chloropentafluoroethane (F115, whoseaction on ozone is accompanied by a very strong contribution towards thegreenhouse effect) and, in the near future, F22 orchlorodifluoromethane. In this respect, it forms part of the compositionof several mixtures of quasiazeotropic nature, such as R407 C (mixturewith HFC-125 or pentafluoroethane in a proportion of 50%/50 by weight)or R410 A (HFC-32/HFC-125 or pentafluoroethane/HFC-134a or1,1,1,2-tetrafluoroethane mixture in a proportion of 23%/25%/52% byweight), which are used in the refrigeration industry.

[0003] F32 can be obtained by fluorination of methylene chloride(CH₂Cl₂) using hydrogen fluoride (HF) in the presence of a catalyst, orby hydrogenolysis of dichlorodifluoromethane (F12) orchlorodifluoromethane (F22), or alternatively by decomposition, in thepresence of HF, of α-fluoro ethers under the action of Lewis acids.

[0004] Some of these processes require acidic or basic washes whichintroduce larger or smaller amounts of water into the final product.This product must thus undergo an additional drying operation in orderto satisfy the specifications normally set for hydrofluorocarbons(HFCs), i.e. less than 10 ppm water. Such a specification is required inorder to avoid problems of corrosion in refrigeration machines.

[0005] Molecular sieves, also known as synthetic zeolites, are chemicalcompounds widely used in the industry as adsorbing agents, in particularfor drying gases or liquids. There are metallic aluminosilicates with athree-dimensional crystal structure consisting of an assembly oftetrahedra. These tetrahedra are formed by four oxygen atoms whichoccupy the peaks, and which surround either a silicon atom or analuminium atom placed at the centre. These structures generally containcations to make the system electrically neutral, such as those derivedfrom sodium, potassium or calcium.

[0006] In the case of molecular sieves, of the so-called A type, thetetrahedra are assembled such that they compose a truncated octahedron.These octahedra are themselves arranged in a simple cubic crystalstructure, forming a network with cavities approximately 11.5 Å indiameter. These cavities are accessible via apertures, or pores, whichcan be partially blocked by means of cations. When these cations arederived from sodium, these cavities have an aperture diameter of 4.1 Å,and this thus gives a so-called 4 A molecular sieve. The crystalstructure of such a sieve can be represented by the following chemicalformula:

Na₁₂[(AlO₂)₁₂ (SiO₂)₁₂].X H₂O

[0007] in which X, which represents the number of molecules of waterforming part of the structure (water of crystallization), can be up to27, which represents 28.5% by weight of the anhydrous zeolite.

[0008] After removal of the water of crystallization by heating to atemperature of about 500 to 700° C., the cavities in these substancesare available for the selective adsorption of various gases or liquids.Thus, the pores in the various types of zeolite allow passage andadsorption, in the corresponding cavities, only of molecules whoseeffective diameter is less than or equal to the effective diameter ofthe pores. In the case of the drying of gases or liquids, it is thuswater molecules which are retained by selective adsorption inside theabovementioned cavities, while the substance to be dried is itself notor only negligibly adsorbed.

[0009] The size of the apertures (or pores) can, moreover, be modifiedaccording to the different types of molecular sieve. Thus, by exchangingmost of the sodium ions of a 4A molecular sieve for potassium ions, the3A molecular sieve is obtained, the pores of which have a diameter ofabout 3 Å. The 5A molecular sieve is prepared by replacing the sodiumions with calcium ions, the effective diameter of the pores then beingabout 5 Å.

[0010] Sieves of 3A, 4A or 5A type are widely commercially available.

[0011] In practical terms, the molecular sieves can be combined withother substances such as binders, in particular clays, and thecompositions obtained are shaped, for example, into granules, beads orextrudates.

[0012] The molecular sieves thus conditioned are used industrially byloading into drying columns, into which the wet gas is introduced, andfrom which it emerges dried.

[0013] After a certain period of running in a drying column, whichvaries with the operating conditions (flow rate of gas to be dried,amount of molecular sieve), an increase in the water content of thedried gas leaving the column is observed. This moment corresponds to theobtainment of the water-saturation capacity of the sieve feed stock,i.e. the maximum amount of water which can be adsorbed. This amount isgenerally about 20% by weight, expressed relative to the weight of drysieve.

[0014] The sieve feed stock thus saturated with water must then besubjected to a so-called regeneration treatment, after which the initialcapacity of the sieve to adsorb water is restored. This treatmentusually consists in passing a stream of an inert gas, at a temperatureof between 200° C. and 300° C., into the column. In practical terms,this treatment of the saturated sieve feed stock is carried out in thesame column as that in which the stream of gas to be dried wasintroduced. The same drying column thus functions occasionally in aphase of drying the wet gas, and occasionally in a phase of regeneratingthe molecular sieve feed stock with the inert gas. However, after acertain number of these drying-regeneration cycles, an irreversibledecrease in the water-saturation capacity of the sieve feed stock isobserved, and it is then necessary to stop running the column so as torenew the sieve feed stock with a fresh feed stock.

[0015] In the present text, the expression “fresh sieve feed stock”means a sieve feed stock which has not been used as a drying agent.

[0016] Under the conditions of the industrial practice of drying gasesusing molecular sieves, 2 drying columns are usually used, which can runalternately, one being in the drying phase while the other is in theregenerating phase.

[0017] The drying of F32 with molecular sieves poses a specific problemon account of the proximity of effective diameter between the moleculesof F32 and of water (0.33 nm and 0.21 nm respectively).

[0018] Thus, patent application FR 2,705,586 clearly mentions theplacing in contact, in a pressurized container, of wet F32 with a 3Atype molecular sieve and an ester oil at a temperature of 120° C.

[0019] However, that document teaches that, under these conditions, theF32 is adsorbed onto the said sieve and undergoes a decompositionreaction, the effect of which is, via a modification of the sieve'scrystal state, to greatly reduce its water-saturation capacity.

[0020] That document concludes that such a sieve is not suitable for useas an agent for drying F32. The patent application consequentlyrecommends, with the aim of drying F32 circulating as a refrigerantinside a refrigeration machine, a molecular sieve obtained by acomplementary treatment of a 3A type sieve which results in a decreasein the size of the pores.

[0021] It has now been found that this drawback can be avoided by dryinga stream of F32 produced continuously, within a specific temperaturerange, and by carrying out, in particular, a specific process forregenerating the sieve feed stock.

[0022] One aim of the present invention is thus to propose a process fordrying wet F32, using a simple, commercially available molecular sievewhich can be used in a plant for the industrial production of F32.

[0023] Another aim of the invention is to propose a process forcontinuously drying wet F32 which results in selectively separating thewater from the F32, with reduced losses of F32.

[0024] Another aim of the invention is to propose a process forcontinuously drying wet F32, which comprises a step for regenerating themolecular sieve feed stock which keeps its water-saturation capacitymore or less constant.

[0025] Another aim of the invention is to propose a process forcontinuously drying wet F32, which allows a reduction in the time forwhich the drying columns are stopped in order to renew the molecularsieve feed stock.

[0026] It has now been found that the above-mentioned aims are achieved,partially or totally, by means of the process according to the inventionwhich is described below.

[0027] The present invention thus relates to a process for drying wetF32, which comprises placing a stream of the said F32 in continuouscontact with a feed stock of a composition comprising a molecular sievechosen from a 3A, 4A or 5A type sieve, at a temperature of between 5 and78° C., preferably at room temperature, and at a pressure of between 0.6and 25 atm, preferably between 0.8 and 17 atm.

[0028] In contrast with the teaching of the prior art, it is thuspossible, in accordance with the invention, to use A type sieves, whichare commercially available, to dry F32 continuously.

[0029] The stream of F32 to be dried can be a stream of gas or liquid.When the stream of F32 to be dried is liquid, the process isadvantageously performed at a pressure of between 9 and 25 atm,preferably between 12 and 17 atm.

[0030] When, according to a preferred variant, the stream of F32 to bedried is a gas, the process is performed at a pressure of between 0.6and 10 atm, preferably between 0.8 and 5 atm.

[0031] The stream of F32 to be dried generally comprises a water contentof less than 10,000 ppm, preferably less than 6000 ppm.

[0032] The wet F32 is preferably placed in contact with the sieve feedstock in a drying column located in the downstream part of a plant formanufacturing F32.

[0033] Before using it for drying the stream of F32, the fresh molecularsieve feed stock is subjected to an activation treatment. The aim ofthis treatment is to remove the water adsorbed after the manufacture ofthe material during its storage and the manipulations preceding itsinstallation in the drying column. This treatment generally comprisesheating to a temperature of between 200 and 300° C. and at a pressure inthe region of atmospheric pressure.

[0034] The flow rate of the stream of F32 to be dried and the amount ofsieve feed stock suited to the drying operation can be determinedwithout excessive difficulty by a person skilled in the art who iscompetent in chemical engineering, by means of calculation and tests, asa function of the size of the industrial plant.

[0035] According to a preferred variant of the process according to theinvention, the molecular sieve used is a 3A type sieve. On account ofits effective pore diameter, such a sieve advantageously has a reducedcapacity for adsorbing F32 and improved efficacy.

[0036] According to a preferred variant of the process according to theinvention, the molecular sieve feed stock used is advantageouslyregenerated (after it has reached its water-saturation capacity) by theprocess which consists in heating the said feed stock to a temperatureof between 120° C. and 300° C., preferably between 150° C. and 250° C.,at an absolute pressure of less than 100 mm Hg, preferably less than 80mm Hg. The duration of this process is advantageously determined so asto desorb virtually all of the amount of products (essentially the waterand, to a minor amount, the residual F32) which are adsorbed afterdrying the wet F32. This amount is denoted by the term “initial amount”in the lines hereinbelow.

[0037] According to another preferred variant of the process accordingto the invention, the molecular sieve feed stock used is regenerated bythe process which consists in passing a stream of an inert gas, such ashelium, over the said feed stock, at a pressure in the region ofatmospheric pressure, by working firstly:

[0038] (i) at a temperature at least between 70° C. and 170° C.,preferably between 80° C. and 165° C., for the time required to removeat least 80%, preferably at least 90%, of the initial amount of F32adsorbed in the feed stock, and then

[0039] (ii) at another temperature of between 180° C. and 300° C.,preferably between 190° C. and 250° C., for the time required to removeat least 90%, preferably at least 95%, of the initial amount of wateradsorbed in the feed stock.

[0040] The running time required at the temperature (i) is determined bymonitoring the profile of the content of F32 in the inert gas, leavingthe regeneration column, by suitable control methods, such as bychromatographic assay. The running time required at the temperature (ii)is determined in a similar manner, for example using a humidity meter.These times are based on a certain number of parameters which depend onthe plant and which are well known to those skilled in the art: flowrate of the inert flushing gas, heat of desorption of the water and ofthe F32, calorific mass of the sieve and of the metallic apparatuscontaining the sieve.

[0041] These last 2 embodiments of the process according to theinvention, relating to the methods for regenerating the sieve feedstock, are particularly advantageous since they make it possible, afterthe regeneration, to keep the water-saturation capacity of the molecularsieve feed stock at a value which is more or less equal to that beforeregeneration. Thus, the same sieve feed stock used industrially can beused effectively in a larger number of cycles: drying ofF32/regeneration. Among these two variants, the one using a stream ofinert gas is more particularly preferred since it is simpler toimplement and run in an industrial plant.

[0042] When the treatment to regenerate the molecular sieve feed stockis carried out by means of the 2-step process which has just beendescribed, it is particularly advantageous to carry out step (i) byfirst working:

[0043] (i1) at a first temperature of between 70° C. and 130° C.,preferably between 100° C. and 125° C., for the time required to removeat least 60%. (preferably at least 70%) of the initial amount of F32adsorbed, and then

[0044] (i2) at a second temperature of between 130° C. and 170° C.,preferably between 145° C. and 165° C., for the time required to removeat least 80%, preferably at least 90%, of the initial amount of F32adsorbed.

[0045] Such a treatment allows even better maintenance of thewater-saturation capacity of the sieve feed stock. It also allowsrecovery of F32 whose water content is considerably lower than that ofthe wet F32 to be dried, in particular after step (ii).

[0046] The regeneration treatment for the sieve feed stock, inaccordance with one of the two variants described above, isadvantageously carried out in the same column as that mentioned above.Even more advantageously, the drying process according to the inventionis carried out in two columns in parallel, one running in the phase fordrying the actual wet F32, the other running in the phase forregenerating a saturated molecular sieve feed stock.

[0047] In the case in which, as recalled above, the process is performedfor the regeneration of the sieve feed stock at a heating temperature ofbetween 200 and 300° C. (in the presence of a stream of inert gas),degradation of the water-saturation capacity of the molecular sieve feedstock is observed. Such a degradation would lead to stoppage of theindustrial plant, in order to renew the sieve feed stock, underconditions which are incompatible with the running of an industrialdrying plant.

[0048] Besides the molecular sieve, the composition used in the processaccording to the invention comprises additives which are generally usedin this field, in particular a clay-based binder which allows the shapedzeolite products to retain their ability to be shaped and theirstrength. The composition is generally in the form of pearls, orgranules. With regard to their strength and their effective desiccatingpower, it is desirable for the granules to be essentiallycylinder-shaped, to have a diameter of from 0.5 to 5 mm and a length offrom 3 to 15 mm, and for the pearls to have a diameter of from 1 to 5mm.

[0049] The examples which follow are given purely for the purpose ofnon-limiting illustration of the process according to the invention.

EXAMPLE 1

[0050] Drying of a Stream of F32 with a Feed Stock of 3A Type MolecularSieve

[0051] A feed stock of 40.8 g of Ceca NK 30 (3 Å) sieves, in the form ofgranules with a diameter in the region of 1.5 mm and a length of between5 and 10 mm, is placed inside a stainless steel drying tube (6), with aninside diameter of 14 mm and a height of 750 mm. The drier thus has aworking height of about 380 mm, and it is equipped with a jacket forheating the sieve feed stock.

[0052] The feed stock preactivation treatment is carried out by heatingto 200° C.

[0053] A stream of F32 gas containing 4100 ppm of water is thencirculated through this drying tube at a flow rate of 44 l/h, at atemperature of about 20° C. and at a pressure of 1 atm, and the efficacyof the drying is monitored by measuring the water content, this beingcarried out using an electrical conductivity cell (7) coupled to aconductimeter (8) which is itself connected to a recorder (9) and anautomatic stopping device (10).

[0054] The dry gas then passes through a buffer reservoir with a volumeof 5 litres (15), from which it is sent, with the aid of a membrane pump(1), to a humidifier composed of a column of glass beads (3) and aplunger (4) which introduces liquid water into the system at a flow rateof 0.4 ml/h, such that the stream of dried F32 is again humidified tothe above-mentioned value of 4100 ppm of water. After thishumidification and passage into a homogenization tank (5), the streamreturns to the drier (6).

[0055] A 10-litre buffer tank (11) allows the pressure of the stream ofF32 gas to be maintained at a value in the region of 1 atm.

[0056] The assembly described in FIG. 1 thus constitutes a gas-phasedrying loop which simulates the running of a drying column forcontinuously treating a stream of wet F32 gas.

[0057] A water content for the F32 of less than 10 ppm is measured atthe drying tube outlet.

[0058] The deviation from the output signal of the cell, indicating thatthe water-saturation capacity of the sieve feed stock has been reached,occurs after running for 18 h 30, which corresponds to awater-saturation capacity of 19.9% relative to the weight of dry sieve.

EXAMPLE 2

[0059] Regeneration of the Feed Stock of 3A Type Molecular Sieve byHeating at 200° C. and at a Pressure of 1 mm Hg:

[0060] The assembly described in FIG. 1 also makes it possible to carryout several cycles for the same molecular sieve feed stock; each cyclecomprises the continuous drying of wet F32 to the point ofwater-saturation of the sieve feed stock, followed by regeneration ofthe said feed stock. These cycles are carried out with a minimumconsumption of F32.

[0061] After running for 18 h 30 and reaching the water-saturationcapacity of the sieve feed stock as described in Example 1, circulationof the stream of wet F32 gas is stopped by closing the appropriatevalves.

[0062] A stream of helium is then circulated, for 2 hours at roomtemperature, in the drying tube (6), the aim of this operation being toremove the F32 remaining between the granules of the molecular sievefeed stock.

[0063] Valves (18) and (19) are thus closed and valve (22) is opened, soas to connect the drying tube (6) to a vacuum pump, via a metal trapimmersed in liquid nitrogen.

[0064] The pressure in the said tube is thus lowered to a value of 1 mmHg. The temperature in the drying tube (6) is set at 200° C. bycirculating a heat-conducting fluid in the jacket of the said drier.

[0065] These temperature and pressure conditions are maintained forabout 2 hours, until complete desorption of the water and of the smallamount of F32 still adsorbed in the sieve feed stock. The water (and theF32) thus desorbed are retained in the liquid nitrogen trap, the weightof which is determined at regular time intervals. The regenerationtreatment is stopped when the weight of the trap is more or lessconstant.

[0066] The drying test as defined in Example 1 is then repeated with thesieve feed stock thus regenerated.

[0067] The water-saturation capacity of the sieve feed stock is reachedafter running for 19 hours. It is 18.4%, and thus represents 92.5% ofthe water-saturation capacity determined at the end of Example 1.

[0068] This example thus shows that the water-saturation capacity ismaintained at a more or less constant value after the regenerationtreatment, which is advantageously included in the process for dryingwet F32 according to the invention.

EXAMPLE 3

[0069] Regeneration of a Feed Stock of 3A Type Molecular Sieves with aStream of Helium, Carried Out with 3 Steady Temperature Regimes:

[0070] Example 1 is repeated with a fresh feed stock of 41.1 g of CecaNK 30 (3 Å) molecular sieves.

[0071] The saturation capacity is reached after running for 19 hours. Itis 18.5% relative to the weight of dry sieves.

[0072] The sieve feed stock is then regenerated by circulating a streamof helium in the drying tube (G), at normal atmospheric pressure andunder the following conditions:

[0073] at 120° C. for 2 hours, then

[0074] at 150° C. for 1 hour 30, then

[0075] at 200° C. for 2 hours.

[0076] Chromatographic monitoring (14) of the desorbed F32 shows thatthe corresponding amounts of F32 (expressed relative to the initialamount of F32 adsorbed) are about 70% after the steady regime at 120° C.and about 90% after the steady regime at 150° C.

[0077] Chromatographic monitoring (14) of the water in the stream of Heshows that, after the steady regime at 200° C., more than 95%. of thewater adsorbed onto the sieve feed stock has been desorbed.

[0078] The drying test as defined in Example 1 is then repeated with thesieve feed stock thus regenerated.

[0079] A water-saturation capacity of 16.9% is measured. Such a valuecorresponds to 91.3% of the water-saturation capacity achieved at theend of the drying step of the present example.

[0080] This example thus shows that the water-saturation capacity ismaintained at a more or less constant value after the regenerationtreatment, which is advantageously included in the process for dryingwet F32 according to the invention.

EXAMPLE 4

[0081] 40.3 g of a sieve feed stock which is not fresh are used, thewater-saturation capacity of these sieves, determined previouslyaccording to Example 1, being 14.2%.

[0082] A series of drying/regeneration cycles is carried out using thissieve feed stock; each cycle comprises the continuous drying of the wetF32, carried out in accordance with Example 1 to the point ofwater-saturation of the feed stock, and followed by regeneration of thesaid-feed stock in accordance with the following temperature profile:

[0083] 3 hours at 120°

[0084] 3 hours at 160°

[0085] 2 hours at 200°.

[0086] It is found that 98% of the initial amount of F32 adsorbed isdesorbed after 3 hours at 160° C.

[0087] The results are collated in the following table. They show thatthe water-saturation capacity of the sieve feed stock is kept more orless constant. Water capacity Cycle No. (%) 1 14.2 2 14.3 3 14.5 4 13.05 15.3

COMPARATIVE EXAMPLE

[0088] Example 1 is repeated with a fresh feed stock of 40.8 g ofsieves.

[0089] The water-saturation capacity is reached after running for 19 h.It is 18.4% relative to the weight of dry sieves.

[0090] The regeneration is carried out while flushing with dried helium(12) preheated to 150° C. (oven 13), the bed of sieves beingsimultaneously heated via the drier (6) jacket; the aim of these methodsis to very rapidly reach a temperature of 200° C. on all of the sievefeed stock, as occurs in practice in an industrial process.

[0091] After a steady regime of 2 hours at this temperature of 200° C.,the sieve is cooled and 35.3 g of this feed stock are subjected to thedrying test as defined in Example 1.

[0092] The water-saturation capacity of the sieve feed stock is reached,in this case, after running for 10 hours and is only 11.2%. This valuecorresponds to a 40% decrease relative to the initial saturationcapacity, before regeneration.

1. Process for drying wet F32, which comprises placing a stream of thesaid F32 in continuous contact with a feed stock of a compositioncomprising a molecular sieve chosen from a 3A, 4A or 5A type sieve, at atemperature of between 5 and 78° C., preferably at room temperature, andat a pressure of between 0.6 and 25 atm, preferably between 0.8 and 17atm.
 2. Process according to claim 1, characterized in that the streamof F32 to be dried is a stream of gas, and the pressure is between 0.6and 10 atm, preferably between 0.8 and 5 atm.
 3. Process according toeither of claims 1 and 2, characterized in that the stream of F32comprises a water content of less than 10,000 ppm, preferably less than6000 ppm.
 4. Process according to one of claims 1 to 3, characterized inthat the wet F32 is placed in contact with the sieve feed stock in acolumn located downstream of a plant for manufacturing F32.
 5. Processaccording to one of claims 1 to 4, characterized in that the molecularsieve used is a 3 A type sieve.
 6. Process according to one of claims 1to 5, characterized in that the sieve feed stock is regenerated by theprocess which consists in heating the said feed stock to a temperatureof between 120° C. and 300° C., preferably between 150° C. and 250° C.,at an absolute pressure of less than 100 mm Hg, preferably less than 80mm Hg.
 7. Process according to one of claims 1 to 5, characterized inthat the sieve feed stock is regenerated by the process which consistsin passing a stream of an inert gas, such as helium, over the said feedstock, at a pressure in the region of atmospheric pressure, by workingfirstly: (i) at a temperature at least between 70° C. and 170° C.,preferably between 80° C. and 165° C., for the time required to removeat least 80%, preferably at least 90%, of the initial amount of F32adsorbed in the feed stock, and then (ii) at another temperature ofbetween 180° C. and 300° C., preferably between 190° C. and 250° C., forthe time required to remove at least 90%, preferably at least 95%, ofthe initial amount of water adsorbed in the feed stock.
 8. Processaccording to claim 7, characterized in that step (i) is carried out byfirst working: (i1) at a first temperature of between 70° C. and 130°C., preferably between 100° C. and 125° C., for the time required toremove at least 60% (preferably at least 70%) of the initial amount ofF32 adsorbed, and then (i2) at a second temperature of between 130° C.and 170° C., preferably between 145° C. and 165° C., for the timerequired to remove at least 80%, preferably at least 90%, of the initialamount of F32 adsorbed.
 9. Process according to one of claims 6 to 8,characterized in that the regeneration treatment for the sieve feedstock is carried out in the same column as that defined in claim
 4. 10.Process according to claim 9, characterized in that it is carried out intwo columns in parallel, one running in the phase for drying wet F32,the other running in the phase for regenerating a saturated molecularsieve feed stock.